Conventional laser systems are typically employed for processing target structures such as electrically conductive or resistive films in integrated circuits on silicon wafers. Trimming film capacitors, inductors, or resistors, which is presented herein only by way of example of selective material processing, may include total or partial removal or vaporization of the film material. Examples of resistive film trimming and functional device trimming laser systems include Model Nos. 4200, 4400, and 6100, manufactured by Electro Scientific Industries, Inc., which is the assignee of the present application. These systems typically utilize output wavelengths of 1.064 .mu.m, 1.047 .mu.m, and 0.532 .mu.m.
FIG. 1A is a plan view of a portion of integrated circuit 10 depicting resistors 12a and 12b (generally, resistor 12) having a patterned resistor path 14 between metal contacts 16. The resistive value of a resistor 12 is largely a function of the pattern geometry, the path length between contacts 16, and the thickness of material composing resistor 12.
"L-cut" 15 on resistor 12a depicts a typical laser trim. In an L-cut 15, a first strip of resistive material is removed in a direction perpendicular to a line between the contacts to make a course adjustment to the resistance value. Then an adjoining second strip, perpendicular to the first strip, may be removed to make a finer adjustment to the resistance value. "Serpentine cut" 17 on resistor 12b depicts another common type or laster trim. In a serpentine cut, resistor material is removed along lines 18 to increase the length of path 14. Lines 18 are added until a desired resistive value is reached.
FIG. 1B is a cross-sectional side elevation view depicting a conventional output energy distribution of a laser output or pulse 20 directed at a resistive film structure 22 such as resistor 12. With reference to FIGS. 1A and 1B, resistive film structures 22 typically comprise a thin film layer 24 of a resistive material such as nichrome, tantalum nitride, cesium silicide, or silicon chromide, that is layered upon (or embedded within) a substrate 26 such as silicon, gallium arsenide, or other semiconductor materials. Alternatively, thin film layer 24 may be applied to an epitaxial or junction layer 28, such as an oxide of substrate 26. Thin film layer 24 may be covered by a protective layer 30, such as a dielectric, either due to an IC processing requirement or to contain laser trimming by-products or slag from splattering other integrated circuit elements. Integrated circuit 10 and resistive film structure 22 may also be composed of several materials including those required for anti-reflective coating, binding, or other manufacturing purposes.
If the film is a conductive film and part of an electrode of a capacitor device, the effective area of the film can be changed by punching holes in or trimming the film to reduce the capacitance of the film until it reaches a predetermined capacitance value.
The process of trimming conductive or resistive films to provide integrated circuit elements having predetermined capacitive or resistance values without powering up or operating the circuit is commonly referred to as "passive trimming." This process may involve measuring the circuit element during or following each trimming operation and to cease trimming when the predetermined value is obtained.
With reference to FIG. 1B, resistive film trimming may be performed with a conventional laser system. Laser pulse 20 is focused to a spot of radius R (which is, for example, about 2 to 5 .mu.m) and applied in a predetermined path 14 across resistive structure 22 and has a suitable duration or pulse width and sufficient energy at a conventional wavelength to vaporize thin film layer 24. Laser pulse 20 must, however, be tailored to limit energy so as not to cause damage to substrate 26 or adjacent circuit structures either by direct laser energy absorption, by residual pulse energy coupled into substrate 26 below film layer 24 after it is vaporized, or by thermal conduction.
Simply decreasing the laser output power level often results in inadequate film vaporization and unclean kerfs or trims, exhibiting striations or poorly defined edges, that may increase the noise characteristics or jeopardize the stability of an integrated circuit.
Various laser pulse widths, repetition rates, and scanning speeds have been tested to optimize the trimming results for different film structures and materials and have achieved a certain level of success.
"Functional" laser processing or trimming of components in active or dedicated devices is yet another example of selective material processing. Such devices may, for example, be totally silicon wafer-based devices or hybrid integrated circuits (HIC) including some silicon wafer-based devices. In functional trimming, the device is powered up and then its performance is evaluated. Then, certain component(s), such as capacitors, resistors, or inductors, of the device are trimmed. Evaluation of the device function and trimming of the device components are repeated incrementally until the device performs to specification. Functional trimming is described in detail by R. H. Wagner, "Functional Laser Trimming: An Overview," Proceedings of SPIE, Vol. 611, Jan. 1986, at 12-13, and M. J. Mueller and W. Mickanin, "Functional Laser Trimming of Thin Film Resistors on Silicon ICs," Proceedings of SPIE, Vol. 611, Jan. 1986, at 70-83.
Functional trimming with conventional laser outputs such as 1.064 .mu.m, 1.047 .mu.m, or their harmonics presents an additional problem to that associated with passive trimming. These laser wavelengths tend to cause opto-electric responses such as generation of excessive carriers (electron-hole pairs) in the semiconductor substrate material which can affect the device and may result in performance drift or complete malfunction of the device or circuit. Furthermore, scattered laser light may impinge on adjacent active devices (such as P-N junctions or field effect transistors (FETs) or any semiconductor material based structures) and affect their performance due to excitation of carriers in the structure, resulting in performance drift or malfunction of the device or circuit during active trimming.
Thus, functional laser processing with these wavelengths is impossible or prohibitively slow because extra time is required to let these carriers disappear before the device or circuit will function normally. This is also true for integrated circuit (IC) or hybrid integrated circuit (HIC) devices having photo-receptive or light-sensitive portions or photo-electronic components such as photodiode or charge-coupled device (CCD) arrays integrated as part of the devices' function logic. During laser processing with conventional laser wavelengths, semiconductor substrate materials, semiconductor materials-based structures such as P-N junctions or FETs, as well as photo-receptive, light-sensitive or photo-electronic components or portions would react to the laser energy, causing the entire device to malfunction, regardless of their proximity to the laser beam. Functional laser processing or trimming of such devices, where the processing control is based upon measurement values obtained during real-time function of the device, is impossible or extremely slow to perform because of laser-induced malfunctions of the devices when using conventional laser processing wavelengths.